1. Field of the Invention
The invention relates to helicopters and more particularly to a system for individually controlling the pitch angles of a four-blade rotor.
2. Description of the Prior Art
A helicopter is supported by a rotor having a plurality of articulated blades, free for up and down motion (flapping), lead and lag (in the plane of motion), and along the axis of the blade (pitch or feathering). The horizontal motion of the craft is controlled by tilting the plane of the rotor, which in turn results from controlled feathering of the rotor blades, while flapping and lead-lag result from aerodynamic and centrifugal forces.
Pitch of the blades in the prior art is controlled by linkages to a rotating plate (swashplate) which is axially inclined and bears against a nonrotating plate whose plane is tilted by three servo actuators in response to the pilot's cyclic stick and collective stick controls.
For optimum performance it is desirable that the pitch of each blade be individually controllable as it rotates in azimuth. This has been achieved in a three-blade rotor, but not in a four-blade rotor.
Individual blade control (IBC) for helicopters has been recognized as promising improvements in performance in the presence of wind gusts, reduced vibration, and flying qualities. In one proposed implementation, the swashplate is replaced by individual blade actuators placed in the rotating system. For safety, a high degree of redundancy is required and reliable means for transmitting control signals to the rotating actuators have yet to be developed. In another proposed embodiment, inputs are transmitted through a conventional swashplate at predetermined harmonic control frequencies. However, for rotors comprised of more than three blades, certain control frequencies cannot be independently passed to each blade, thus negating many of the potential advantages of IBC.
The present invention provides a simple, mechanically reliable apparatus for achieving individual blade control without requiring development of high technical risk components. Individual blade control is introduced by providing an independently actuated differential sleeve to the normal swashplate blade control system.